1. Field of the Invention
The present invention relates to an improved retarding field optical apparatus for focusing and deflection of high resolution, high-current, low-energy charged particle beams. Additionally, the present invention relates to apparatus for detecting the charging currents produced by such a beam when the beam interacts with a target, the detection apparatus being physically and electrically integrated with the retarding field apparatus so as to provide a system providing excellent signal detection and improved optical performance.
More particularly, a preferred embodiment of the present invention relates to a method and system for using an electron beam to test conductive networks in multi-chip modules (MCMs) for such defects as opens between networks which should be electrically connected and shorts between networks which should be electrically isolated, using an electron beam capable of large field deflections at low voltages.
2. Description of the Related Art
There are many important applications for high-resolution, high-current, low-voltage charged particle beams including, for example, applications in microscopy, surface analysis, integrated circuit testing, and lithography.
Within the field of electron beam column technology, which is concerned with apparatus for the production, focussing, and deflection of beams of electrons or other charged particles, the difficulty of producing low-voltage beams by conventional techniques is well known. Factors contributing to these difficulties include: i) the proportionality of source brightness to beam voltage; ii) the additional reduction of source brightness at low-voltage due to electron-electron interaction effects and space charge saturation effects; iii) the increased chromatic aberration of lenses and deflectors; and iv) increased beam sensitivity to the disturbing effects of ambient magnetic fields and charging within the column.
Recently, the advantages of employing an unconventional approach to the production of low voltage beams have been recognized. The new approach involves accelerating the beam to a kinetic energy that exceeds the desired landing energy at the target and subsequently decelerating or retarding the beam to the desired impact kinetic energy. This new approach is termed "a retarding field technique" (RFT).
The RFT has two primary advantages. First, the stronger accelerating field at the electron source improves source brightness at low voltage. Secondly, the decelerating or retarding field, appropriately designed, can be combined with a conventional lens with the result that the performance of the combination is much better than that of the conventional lens alone.
The optical performance advantages of the RFT are only fully realized when the retarding field immediately precedes the target. Such a configuration, consists of a conventional lens (either magnetic or electrostatic), a retarding field (electrostatic) and then the target. For a probe forming system, the configuration normally must also include means for deflecting the beam. Such a configuration, including deflection means, is termed "a retarding field objective" (RFO).
While the optical performance advantages of RFOs are well known, a problem remains which limits their usefulness. In most applications, one is interested not in the primary beam itself, but in its interaction with a target. Unfortunately combining RFOs with conventional charged particle detectors, which collect electrons emitted because of the beam-sample interaction, is difficult.
The problem arises since the retarding field, which decelerates the primary electrons, is an accelerating field for the electrons emitted from the sample. This acceleration confines the emitted electrons to trajectories which are physically close to the primary beam trajectory. The narrow emission angle of the emitted electrons and their proximity to the primary beam electrons make their detection difficult, since the detector must be simultaneously transparent to primary beam electrons and somehow collect emitted electrons. Further, the collector must be designed so as not to adversely affect the electrostatic field within the retarding field lens.
As a result, as a practical matter, the detector must be outside the retarding field, which means overall system length must be increased to accommodate it. Increased system length decreases the optical performance.
Thus, the retarding field allows production of a better primary beam but simultaneously limits or compromises the ability to determine the sample's features with this beam using conventional electron detectors.
Heretofore the present invention, therefore the design of retarding field objective lenses was driven by a compromise between good performance for primary beam production and good performance for emitted electron detection.
These compromises are well illustrated in a conventional design in which to separate the incoming primary beam electrons and the outgoing emitted electrons, an additional optical element is required. More specifically, a bending magnet, which directs the emitted electrons away from the primary beam so that they can be detected by conventional detectors physically distant from the primary beam, has been required.
This approach has several problems. In addition to the complexity and expense of the bending magnet, there are two additional problems.
First, the bending magnet degrades primary beam resolution. Secondly, the bending magnet rigidly constrains the design of the beam deflection system. The deflection system must not only deflect the primary beam on the target, but simultaneously deflect the emitted electrons so that they travel backwards along the axis of the system so as to enter the bending magnet. This constraint on the deflection system limits the optical performance by restricting the deflection elements to electrostatic elements and limiting how they may be positioned and energized.
Further, it is well known that contactless test systems are needed for the substrates of multi-chip-modules. In the production of multi-chip modules (MCMs), the component parts of the modules should be tested for defects before assembly to minimize the cost of repairing such defects and to maximize the yield of operable devices.
Component testing includes testing for and the detection of opens and shorts in the conductive networks of the substrate on which the integrated circuits are mounted. Mechanical probe systems are commonly used for this purpose, but these systems oftentimes damage substrates, add particulate contamination, have limited throughput, and are not applicable to feature sizes below approximately 25 micrometers, or to small features recessed into insulators because of these problems, non-contact techniques have been investigated for open and short defect detection.
Among the non-contact test systems are electron beam systems which use voltage contrast. However, voltage contrast systems have several drawbacks including the propensity to detect "false" shorts, i.e., to detect shorts between nets which have an inter-net resistance of 100 megohms or more.
A second drawback of voltage contrast systems is that primary beam deflection must cover the entire substrate to obtain practical test times.
In view of the foregoing problems of the conventional systems, there is a need for a non-contact substrate testing system which overcomes the above-mentioned problems.
A test system and method which overcomes the abovementioned problems have been described in copending U.S. patent application Ser. No. 08/036,781, to Golladay, filed on Mar. 25, 1993, now U.S. Pat. No. 5,404,110, incorporated herein by reference and assigned to one of the co-assignees of the present application.
The method of Golladay '781 uses an induced current signal to detect defects, and is referred to as an induced current test method (ICTM) in which an induced current signal is detected to determine whether defects exist. The advantages of ICTM include: i) its reduced propensity to detect false shorts; ii) the elimination of the need for full substrate deflection; iii) minimal insulator charging effects because of the use of a relatively low primary beam landing energy, eV.sub.L (e.g., typically in the range of 500-800 eV) on the substrate; and iv) its ability to measure network capacitance.
However, while many of the advantages of the ICTM result from using a low voltage beam, the production and deflection of low voltage beams with high beam current and small spot size is difficult, as discussed above. Nevertheless, despite these difficulties, obtaining sufficiently high beam current in a sufficiently small focussed probe which can be deflected electromagnetically over a sufficiently large deflection field is essential to achieving optimum tester performance.
Thus, the conventional systems, i.e. , those not employing RFT, have suffered from not being able to produce a high-resolution, low energy beam with a sufficiently high beam current, nor to deflect a low energy beam over a sufficiently large area, for high throughput production testing due to inherent problems with the conventional approaches.
Moreover, existing systems employing RFT have suffered from the same limitations due to design compromises arising from the incompatibilities of probe formation and emitted electron detection in those systems.